Girdling Tree Roots

Just one girdling root
can kill a tree!

Q. We have a dying maple tree that has been diagnosed
with a "girdling root." We were told to have the tree removed and
plan to do so. This was the first tree we planted when we moved into
our home 20 years ago. We love the maple and hate to lose it, so I
would like to know if there is anything we can do to prevent this
problem the next time we plant a tree. We intend to replace it with
another maple.

A.
Girdling roots are those that wind up growing around the trunk of a
tree rather than out into the surrounding soil. As the girdling root
and the trunk of the tree grow in diameter, the girdling root slowly
chokes off the flow of water and nutrients to the tree. Girdling
roots may be clearly visible above the soil line, but they are often
found below ground.

Symptoms of Girdling Roots

Symptoms include reduced growth, smaller-than-normal leaves,
fewer-than-normal leaves, lighter green leaf color, branch dieback
and the eventual death of affected trees. These symptoms are
frequently seen just on the side of the tree where the offending
root is growing. The trunk may look 'flattened' on that side, too.
Girdling roots do not become obvious until five to 20 years after a
tree is planted, and it happens so gradually that people often do
not realize there is a problem until it is too late.

Certain species of trees are more susceptible than others, including
maple (Acer spp.,
except silver maple), sweet gum (Liquidambar
styraciflua), magnolia, pine and linden (Tilia
spp.). This susceptibility can be compounded by poor site conditions
and poor planting practices. Street trees grown in tree pits that
are too small or trees growing in severely compacted soil are more
likely to have a problem with girdling roots. Trees that have been
planted too deeply or mulched too deeply are also good candidates.

Check Container Grown Trees

Container-grown trees and shrubs that are planted straight out of
the container without breaking their circling roots before planting
are very likely to develop girdling roots, too. On container plants,
score the rootball by cutting an inch deep into it from top to
bottom on at least four sides. You should also make two diagonal
cuts across the bottom of the rootball in an inch-deep "X" pattern.
Untangle badly circling roots by hand, and prune off those roots
that are circling so deeply back into the rootball that you cannot
untangle them. Some arborists recommend removing all artificial soil
from the roots of container-grown plants so you can clearly see the
circling roots and eliminate them.

Trees should always be planted so that the root flare -- where the
trunk flares out to meet the roots -- is just at ground level. If
your tree goes straight into the ground like a telephone pole, it is
planted too deeply. If you are planting a balled-in-burlap tree, do
not dig the hole until you find the root flare so you know how deep
to dig it. Untie the twine that holds the burlap around the trunk
and pull the burlap back so you can clearly see the top of the
rootball (do not remove the burlap completely until you have the
tree situated in the hole).

If you cannot see the root flare, remove soil around the trunk until
you can see it. You can use your hands, a hand cultivator or the
curved part of a wire coat hanger. The root flare can be several
inches deep, so be patient and work gently until you find it. You
will break some fine roots as you do this. They are known as
adventitious roots and can become girdling roots in the future
unless the tree is re-planted at the proper depth.

Planting Hole

Once you find the root flare, dig the hole so that it will be at
ground level once the tree is planted. The hole should be two or
three times as wide as the rootball. Backfill with the soil you dug
out of the hole. Amending the backfill with a lot of organic matter
is no longer recommended.

The above
planting directions apply to container-grown plants, too. The main
difference is that you can build a cone of soil in the bottom of the
hole and splay the slashed rootball over it as additional insurance
against girdling roots. If you do not remove the artificial soil, be
sure it is completely covered with a thin layer of native soil.
Otherwise, that artificial mix will dry out much faster than the
surrounding soil. Thoroughly soak the rootball close to the plant
and the surrounding soil when you water to prevent drought stress.

A major root on this small bonsai tree
illustrates how plastic pots cause tree roots to grow in
a circling pattern.

An inch or two of mulch is sufficient to maintain soil moisture,
moderate soil temperature and help keep weeds down. Mulch should be
placed close to but never touching the trunk and extend out to the
drip line (the ends of the branches). Applying more mulch than that
is like planting a tree too deeply, encouraging adventitious roots
that can become girdling roots. Mulch also becomes water repellent
once it dries out completely as has happened during our recent dry
spell, causing more drought stress. If the mulch is piled up around
the trunk, it can hold too much moisture around the bark during wet
weather, possibly causing it to rot. A
deep layer of mulch also
provides a favorable environment for voles and other rodents to take
up residence. Trees can survive girdling roots if they are caught
early on and removed while they are still small. It is best to have
this done by a certified arborist who has the training to determine
if removing the root would create an unstable tree hazard. Never
attempt to remove very large girdling roots.

Root Crown Excavations

Some arborists also offer root crown excavations to uncover the root
systems of trees that have been planted too deeply or have had
additional soil moved over their roots systems during construction.
Some trees may be too far gone to be worth the investment, but I
have seen trees recover miraculously once their root crowns are
uncovered.

To avoid problems with your new maple, evaluate the planting site
for very compacted soil. Plant the new tree at the proper depth, no
deeper. And remember that more is not better when it comes to mulch.